1. Field of the Invention
The present invention relates generally to the area of agitating and more particularly, to an orbiting platform liquid mixer having reversing orbiting stirring cycles.
2. Description of the Related Art
The mixing of liquids in a laboratory environment must be done in accordance with many different requirements. For example, the mixing of liquids of widely different viscosities may require a vigorous mixing. In contrast, the mixing of cell cultures may require a gentle but thorough mixing. To accommodate the varying needs, there are two basic types of liquid mixers. The first has a stationary base on which the fluid container rests. A stirring bar is inserted into the container, and the stirring bar is coupled by a magnetic field to a magnet located beneath the table. Rotating the magnets beneath the table rotates their coupling magnetic fields and causes a corresponding rotation of the stirrer bars in the liquid. The direction of rotation of the stirring bars may be reversed, and rotation of the stirring bars may be continuous or intermittent such that there is a stirring of the liquids by mixing the liquids with a vortex produced by the stirring bars. Alternatively, the stirring bar rotation may be reversed each half cycle thereby mixing the liquids without a vortex, that is, shaking the liquids, The cycle times for the stirring bar rotation and the rest period between intermittent stirring bar rotations are selectable.
Liquid mixers using magnetic stirring bars have several disadvantages. First, the presence of the stirring bar in the liquid reduces the volume available for the liquids and hence, reduces the yield. Second, the presence of the stirring bars in the liquid has the disadvantage of being a potential source of contamination. Further, the magnetic stirring bars are less effective at keeping cells in suspension; and the abrupt directional reversals of the stirring bars may damage cells in the liquids being mixed. Further, if the magnetic stirring bar hits a projection or other irregularity on the bottom of the container containing the fluids, the resulting stirrer motion is irregular and has the potential of breaking the magnetic coupling between the stirring bar and the rotating magnet. Further, there is always the disadvantage of breaking the magnetic coupling between the stirring bar and the rotating magnet beneath the table thereby interrupting the stirring operation and potentially damaging the liquids being mixed.
Another type of liquid mixer is one in which one or more liquid containers is mounted on a moving platform. Commercially available moving platform liquid mixers provide a wide range of unidirectional rotating or reciprocating motions and more complex shaking motions. Of particular interest is a moving platform mixer having a continuous unidirectional orbiting motion. For purposes of this disclosure, an orbiting motion refers to a continuous circular motion having a diameter in the range of approximately from 0.5 inch to 1 inch. Typically, orbiting platform liquid mixers are powered by an electric motor which is mechanically coupled to the platform to provide the orbiting motion. Alternatively, the motor may be coupled to the platform to provide a reciprocating motion which is mechanically produced from unidirectional motion of the motor. The orbiting or reciprocating motion may be continuous or intermittent. Further, the total mixing time, the platform speed and time period between intermittent operations is adjustable. However, none of the orbiting platform liquid mixers provide the reversing orbiting operating cycle of the present invention.
Prior mixing controls require the entry of a wide range of time and velocity parameters, for example, mixing time may be varied in one minute increments from a minimum value of one minute to a maximum value of 99 hours, 59 minutes. However, the input circuits on most devices do not contain a numeric keypad with which the specific set point times can be entered. The input circuits on those devices only have time-up or time-down keys which are selectively depressed to sequentially increase or decrease the time in one minute increments, So that the full range of the time and velocity parameters can be traversed in a reasonable period of time, the prior controls have the capability of sensing the continuous activation of the increase or decrease keys. Further, the controls will increase the time rate of change of the value of the selected parameter in proportion to the period of time that the key is activated. Usually, there is some maximum limit to the rate of change which is dependent on the ability of an operator to respond to the changing values. As the operator watches the value of the parameter change, the operator is preparing to release the key in response to a displayed value. Consequently, there is a significant probability that given that state of preparedness, one or more unintentional releases may occur. That probability increases if the operator is unfamiliar with the touch or feel of the keys. With an unintentional release, prior mixing controls have the disadvantage of always reseting the rate of change of the value of the parameter back to its slowest: rate of change. Consequently, unintentional releases of the key will result in a significantly longer time period to select the desired value of the parameter.